This invention is in the field of air conditioning, specifically thermally driven air-conditioners that are capable of accepting a solar input.
Solar air conditioning has great potential to reduce energy use from air conditioning. Sunlight is most plentiful in the summer when air conditioning is required.
The problem is that existing solar technologies have not produced systems that are economically competitive with conventional electrically driven systems. Prior work with solar air conditioning has not produced practical systems. Solar air conditioning systems have used two basic approaches in an attempt to capture the sun""s energy for coolingxe2x80x94thermal and photovoltaic.
The photovoltaic systems use photovoltaic panels to convert solar radiation directly into DC electricity. Photovoltaic systems have two major advantageous attributes: they can use conventional electrically driven air-conditioning equipment which is widely available and inexpensive with the addition of the solar panels that use an inverter to produce AC power, and they can use the utility grid for backup power during dark or cloudy periods.
Unfortunately there are other attributes: the high cost of manufacturing, the low conversion efficiencies, and the need for a continual stream of photons to produce power, create three major disadvantages. First electricity from solar cells is very expensive because of the high cost of the solar panels. (Panels for a residential air conditioner can cost well over $10,000.) Second the space needed for powering the air conditioning units is large. And third the panels provide no energy storage, which creates a need for use of grid based electricity at night and on cloudy day. In fact, the peak output from the solar panels occurs around solar noon, while peak air-conditioning load occurs several hours later, resulting in a significant mismatch between supply of needed power and demand. This mismatch greatly reduces the value of the system in reducing peak power demand to the utility, demands which recently deregulated markets is demonstrating are much more expensive to meet than had heretofore been obvious. For off-grid locations, the only viable energy storage system to match the provision of power to times when demand is high (later in afternoon and at night) is batteries. Batteries have a high first cost, require periodic replacement, and normally use toxic and/or corrosive materials. These problems have prevented the use of photovoltaic systems in other than a few high-cost demonstration systems.
Thermal systems use heat from the sun to drive an air conditioner. Typical approaches use a high-temperature flat-plate collector to supply heat to an absorption system. Systems with concentrating collectors and steam turbines have also been proposed. Natural gas or other fuel is used for backup heat. While thermal systems have the advantage of eliminating the need for expensive photovoltaic panels, they have attributes that produce major disadvantages.
One problem is the high cost and large size of the solar collectors. Flat-plate collectors running at about 190xc2x0 F. (90xc2x0 C.) require double-glazing and selective surface to achieve reasonable efficiency levels, which greatly increases the collector cost. This high collector cost reduces the comparative attractiveness of such systems to standard vapor compression systems driven by grid electricity. Large collector size also reduces the potential market size by eliminating many locations from possible use of the systems.
Furthermore, existing thermal technologies also suffer from the poor COP of absorption systems, typically around 0.5. When combined with a typical collector efficiency of 20 to 50%, this inefficiency, besides creating a need for large collector areas, makes the whole system much less economically and environmentally attractive.
Another important problem introduced by the performance attributes of current solar thermal air-conditioning concepts is the high-cost and large size of high-temperature thermal storage. Large thermal storage is required to reduce backup energy (typically gas) that would be used much of the time when their was a mismatch between demand for cooling and solar inputs. This mismatch is the discrepancy between high solar input at noon and large demands for cooling during late afternoon, at night, and on cloudy days. A related problem with existing concepts for thermally driven solar cooling is the need for significant power input for circulating pumps and fans, which further reduces the possible energy savings.
Together these attributes for current concepts for thermally driven solar cooling imply that the large majority of their energy input would come from the backup fuel and electrical input for fans and pumps. In essence, these various problems mean that these solar systems are effectively very expensive gas-driven systems.
No commercially available or conceptually proposed system has been demonstrated that has the attributes that would be needed for commercially viable solar air conditioner. Commercial success will require the system to have the following attributes: low first cost (The market tends to be first cost driven so it is critical that the cost and thus ultimate selling price not be too high.); small collector area (critical to cost and to finding many locations in which installation is practical); small storage size (The mismatch between solar supply and cooling demand requires storage if the system is not to become a glorified means for using fossil energy and if it is to be practical to install in many locationsxe2x80x94as well as low cost to manufacture.); easy to incorporate backup capability (Regardless of storage capacity, the ability of the system to meet demand in extreme and unusual circumstances will be critical to market acceptance; customers demand perfection and then some.).
Evaporative coolers are a related technology with a long history. Direct evaporative coolers are the simplest and most common. They consist of a means for moving air over a wet pad. Water evaporates from the pad and thereby cools and humidifies the air. They are commonly used for comfort cooling in warm, dry climates such as those found in the southwest U.S.
Indirect evaporative coolers are more sophisticated. An indirect evaporative cooler means that air is cooled by contact with a dry surface that is in turn cooled evaporatively.
Desiccant systems dry air for air-conditioning purposes. A typical system uses a solid desiccant impregnated on a wheel of corrugated metal or plastic.
Some more obscure systems appear in the patent literature, but each has its own problems. U.S. Pat. Nos. RE 20,469; 4,660,390; 4,854,129 describe regenerative indirect evaporative coolers that use a portion of the air exiting the dry cooler as inlet air to the wet side. U.S. Pat. No. RE 20,469 describes a cumbersome arrangement of coils and cooling towers this complicated and expensive. U.S. Pat. No. 4,660,390 describes another system that uses tubes in a crossflow configuration to transfer heat between a wet side and a dry side. U.S. Pat. No. 4,854,129 also uses a system that uses a cooling coil with water from a cooling tower.
U.S. Pat. No. 5,050,391 describes another option for the desiccant system. This system uses solid desiccant material and a true counterflow arrangement for the heat exchangers. It also has essentially a single stage of cooling which limits it performance and its ability to use inexpensive desiccant materials.